1. Field of the Invention
The present invention relates to an ink ejector for forming an image on a recording medium such as recording paper by ejecting ink from a number of channels in accordance with a print instruction.
2. Description of Related Art
Of all non-impact printers, ink jet printers have simple principles and can easily perform multiple gradation and colorization printing. Drop-on-demand ink jet printers eject only droplets of ink for printing. Ink jet printers of this type are coming rapidly into wide use because of high ejection efficiency and low running costs.
For example, U.S. Pat. No. 4,879,568, U.S. Pat. No. 4,887,100, U.S. Pat. No. 4,992,808, U.S. Pat. No. 5,003,679 and U.S. Pat. No. 5,028,936, which correspond to Japanese Patent Application Laid-Open No. 63-247051, disclose ink ejectors of the shear mode type for use in xe2x80x9cdrop-on-demandxe2x80x9d printers. Each of the ejectors includes a controller and an ink jet head. The head has actuator walls of piezo-electric material, which are arranged in pairs to define channels between them. The head also has nozzles for the respective channels.
FIG. 9 of the drawings accompanying this specification shows the waveform of voltage for driving the actuator walls of one of the ejectors disclosed in the patents. The waveform includes an ejection pulse for ejecting an ink droplet from each of the associated channels and a non-ejection pulse for canceling the pressure wave vibration in the channel after the ejection. In response to the print instruction for one dot, the associated controller applies an ejection pulse and a non-ejection pulse in that order to the appropriate actuator walls to eject an ink droplet. The ejection and non-ejection pulses have predetermined widths T1 and T3, respectively.
When the ejection pulse rises, electric fields are formed in the actuator walls. The fields enlarge the channel in volume, reducing the pressure in it. Then, ink flows into the channel. In the meantime, the enlargement in volume generates a pressure wave vibration, which develops a pressure. This pressure increases, and reverses the pressure in the channel into a positive pressure, which reaches its peak about when the time T during which a pressure wave is propagated in the channel one-way elapsed after the ejection pulse rises. When this pulse falls, the volume of the channel decreases, developing a pressure, which is added to the pressure having reversed to be positive. The addition develops a relatively high pressure in that portion of the channel which is near to the associated nozzle. This pressure ejects an ink droplet from the channel through the associated nozzle.
When the pressure in the channel reverses substantially from a positive to a negative after an interval T2 from the ejection pulse, the non-ejection pulse rises. The rise of the non-ejection pulse quickly lowers the still positive pressure. When the non-ejection pulse falls, the pressure which has reversed to be negative rises quickly, canceling the pressure vibration. It is therefore possible to prevent accidental ejection of ink droplets (accidental drops), and transfer early to the process according to the next print instruction.
In accordance with the resolution mode specified by print instructions, the voltage for application to the actuator walls is changed to adjust the volume of each ink droplet so as to eject ink droplets each of the volume for the specified resolution. More specifically, the voltage for a normal resolution mode (360xc3x97360 dpi) differs from that for a high resolution mode (720xc3x97720 dpi). The voltage for the normal resolution mode may be 20 volts (E volts) so that the volume of each droplet may range between 30 and 35 picoliters. The voltage for the high resolution mode may be approximately 16 volts (about 3/4 E volts), or be 3 to 4 volts lower than that for the normal resolution mode, so that the droplet volume may range between 20 and 25 picoliters. The dot pitch is changed with the voltage.
Thus, in accordance with the specified resolution mode, the droplet volume is controlled for image formation with the desired dot density on a recording medium.
However, the ratio of the droplet volume in the normal resolution mode to that in the high resolution mode is about 10/7, and therefore the difference in droplet volume is too small. Consequently, the difference in resolution does not make a distinct difference in dot density, that is, clearness or visibility.
In order for the difference in resolution to make the difference in dot density distinct, the difference in voltage may be larger. By way of example, the voltage for the normal resolution mode maybe higher than 20 volts. The higher voltage is preferable because it raises the ejection speed or jet velocity and increases the droplet volume. However, the higher ejection speed amplifies the pressure wave vibration in each channel, amplifying the vibration of the meniscus formed at the front end of the associated nozzle. Excessive vibration of the meniscus may eject ink at wrong times or spatter ink droplets in fine particles, blurring the printing. When a meniscus is formed at the rear end of the nozzle, the associated actuator walls may piezo-electrically deform to lower the pressure in the channel for the next ejection. In this case, the meniscus recedes deep into the channel, forming air bubbles in it, which may render the ejection difficult.
On the other hand, the voltage for the high resolution mode may be lower than about 16 volts to make the droplet volume smaller for higher resolution. The lower voltage lowers the ejection speed and reduces the droplet volume. The ejector forms images while it is moved relative to a recording medium by a carriage motor (not shown). If the ejection speed is low, the ejected droplets may fly in wrong directions onto wrong spots under the influence of wind or the like. In general, while each ejected droplet is flying, it divides into a larger main drop and smaller satellites. If the ejection speed is lower, the main drop and the satellites may fly onto more displaced or dislocated spots, blurring the printing.
Thus, the difference in voltage makes only a limited difference in dot density between the resolution modes.
It is accordingly the object of the present invention to provide an ink ejector for better printing with a more distinct difference in dot density, that is, clearness or visibility.
An ink ejector according to the invention includes an ink jet head for ejecting ink. The head has an ink channel formed therein, which can be filled with ink. The head further has an ink nozzle formed therein and communicating with the channel. The head includes an actuator provided therein for changing the volume of the channel. The ejector further includes a controller for applying at least one ejection pulse of voltage to the actuator in accordance with a print instruction to control the actuator so as to eject ink from the channel through the nozzle. The print instruction includes a setting of resolution with which the ejected ink forms an image. In accordance with the setting, the controller controls the number of ejection pulses of voltage for ejecting ink droplets.
In accordance with a setting of resolution, which represents the clearness of an image to be printed, the controller can change the number of ejection pulses for application to the actuator. If the resolution specified by the user is lower, the controller increases the number of ejection pulses. This increases the frequency of driving the actuator, and therefore the number of ejected ink droplets increases. As a result, the total volume of the droplets increases, and therefore they form a larger spot or area on a recording medium. If the specified resolution is higher, the controller decreases the number of ejection pulses, decreasing the number of ejected ink droplets. Consequently, the droplets form a smaller spot for finer printing. It is therefore possible to make a distinct difference in dot density between settings of resolution.
When the resolution is set up as a first resolution, which may be a normal resolution mode, the controller applies to the actuator two ejection pulses of voltage for printing one dot. When the resolution is set up as a second resolution, which may be a high resolution, the controller applies to the actuator a single ejection pulse of voltage for printing one dot.
By the conventional method of adjusting the resolution by controlling the voltage for application to the actuators, the ratio of the volume of each ink droplet for normal resolution to that for high resolution can be only 10/7. The ejector according to the invention can increase the volume ratio up to 2/1. This makes it possible to make a distinct difference in dot density between settings of resolution. It is therefore possible to achieve printing as the users need.
The second resolution may include a high resolution and a very high (super) resolution. When the very high setting is chosen, the controller applies to the actuator the single ejection pulse for ejecting an ink droplet to print one dot and an auxiliary pulse of voltage for making the droplet smaller. When the very high setting is chosen, it is possible to eject an ink droplet smaller in volume than when the high setting is chosen. This can provide higher resolution as the users prefer and more scope or room for choice of resolution.
After applying the ejection pulse or pulses or the auxiliary pulse to the actuator, the controller may apply to the actuator a non-ejection pulse for varying the volume of the channel to cancel the pressure wave vibration in the channel. The non-ejection pulse prevents the ink head from unintentionally ejecting the ink. Therefore, without waiting until the pressure wave in the channel damps, the controller can quickly apply to the actuator the ejection pulse or pulses for printing the next dot. As a result, it is possible to improve the printing speed.
In this specification, the cancellation of the pressure wave vibration in the channel means not only damping the vibration completely, but also damping it to such a degree that no ink can be ejected.
The non-ejection pulse may have a width between 0.3T and 0.7T or between 1.3T and 1.8T where T is the one-way propagation time which it takes for the pressure wave in the channel to be propagated one way.
The two ejection pulses for printing one dot are a first pulse and a second pulse following the first pulse. The first pulse may have a width between 0.5T and 1.5T. The interval between the first and second pulses, which corresponds to an interval between a falling point (trailing edge) of the first pulse and a rising point (leading edge) of the second pulse may be 0.3T or longer. The second pulse may have a width which is 0.3T or longer. The sum of the interval and the width of the second pulse may range between 1.3T and 1.7T.
The single ejection pulse for printing one dot may have a width between 0.5T and 1.5T.
The controller may include a pulse control circuit. This circuit may include a data receiver, a memory, a processing unit and a pulse generator.